Project a. A comprehensive investigation of major aspects of spinal ganglion development in the chick embryo: Establishment of birthdates of the two populations of neurons in brachial and lumbar ganglia; periods of proliferation and neuron degeneration; counts of neurons, mitotic and degenerating cells in selected stages during these periods. These data will give a picture of the dynamics of attainment of final population size and a basis for b. Project b. The experimental part will be concerned with two aspects. One, the possible role of NGF as a trophic and maintenance agent in normal spinal ganglion development. Injection of labelled NGF into limbs which have acquired sensory innervation will show whether NGF can be transported retrogradely to limb ganglia and, furthermore, whether NGF made accessible by this route will increase proliferation and reduce degeneration numerically. The latter point has not been investigated as yet. Doubts have been expressed that the increase of proliferation induced by systemic injection of NGF or implantation of supernumerary legs produces neurons rather than glia. Increase in neuron population size is not conclusive because this could be due to reduced neuron death. Gliogenesis is supposed to start later than neuron production. NGF-injected embryos will be pulse-labelled during the proliferation phase and sacrificed when neurons and glia can be distinguished. Perhaps early proliferative stages can be found in which neuron production prevails. A case for increased neuron production can be made if in these early stages neuron degeneration is at a low level.